Rod mills of the aforedescribed type are used to roll so-called wire rod, generally in a hot rolling process, following which the rolled product can be drawn through a die to produce wire. The drawing process is usually a cold drawing while the rolling process is usually hot rolling.
The principles of rolling such wire rod are described inter alia, in The Making, Shaping and Treating of Steel, United States Steel Company, Pittsburgh, Pa., 1985, pages 963 ff.
When the initial rolling stages are carried out in common roll stands and the strands are then separated to form the respective loops and are then fed to individual mill stands, the multistrand rolling line can be either a preliminary rolling line, e.g. a roughing mill, or an intermediate line or mill. The single strand series of mill stands can be either intermediate lines or mills, or finishing lines or mills.
As a rule the individual strands are more or less uncoordinated with one another in the rolling operation. As a general matter, they arrive at and leave the mill stands independently of one another and without coordination within the various lines or mills and that applies as well to the entry of a strand to be milled into a common stand or the departure of the end of the strand from a common stand.
However, with each arrival of the leading end of a strand at the common mill stand and with each departure of a trailing end of the rolled stock from such a common mill stand, there is a change in the springing of the frame of the stand, which gives rise to a change in the cross section of the strand which was previously in or was still between the rolls of the mill stand. Because of this change in cross section there is a change in rolling speed. Such speed changes are compensated by the loops which lie downstream of the last common mill stand of the roughing or intermediate mill line or upstream of the individual stand of the subsequent intermediate rolling or finished rolling lines.
The loops have a loop length which can be measured from one end of the loop to the other or simply as the depth of the loop which fluctuates between a minimum value and a maximum value. The minimum value which can correspond to elimination of the loop (minimum value equals zero) occurs as the trailing end of the strand, which has left the common mill stand, is about to enter the individual mill stand. In all cases, therefore, the minimum value of the loop length will be zero. The maximum value of the loop length will depend on the construction of the line and how the latter is operated.
In the normal case the loop length should be midway between the maximum and minimum values and hence approximately half the maximum value. The speed of the mill downstream of the loop can be varied to maintain or restore the loop length at half of the maximum value. Since the maximum speed change arises as the leading end of a strand enters the mill stand or the trailing end of a strand leaves the mill stand, the maximum variation in the loop occurs at these times and by maintaining a half maximum length loop, fluctuations in either direction of the mill stand speed can be readily compensated and effective results from a point of view of the control dynamics can be obtained.